Introduction

This text documents the implementation stages / components for adding SIMD support to GHC for the LLVM Back-end. The overriding requirements and architecture for the project are located back at the ​SIMD LLVM Page.

Based on that design, the high-level tasks that must be accomplished include the following:

Arrange that the other Code Generators continue to generate non SIMD code

Introduction of SIMD support to GHC will occur in stages to demonstrate the entire “vertical” stack is functional:

Introduce “Float” PrimOps (as necessary to run an example showing SIMD usage in the LLVM)

Add appropriate Cmm support for the Float primtype / primop subset

Modify the LLVM Code Generator to support the Double vectorization

Demonstrate the PrimOps and do limited performance testing to ensure SIMD is functional

Modify Vector Libraries to make use of new PrimOps

Modify the DPH Libraries

Higher level examples using the above libraries

Build out the remaining PrimOps

Demonstrate full stack

Test remaining code generators

Current Open Questions

These clearly won't be all of the questions I have, there is a substantial amount of work that goes through the entire GHC compiler stack before reaching the LLVM instructions.

Location of SIMD branch

The SIMD branch of GHC is named, appropriately, simd.

Adding a primtype / primop Outline

When going through this outline, it is helpful to have the ​Compiler Pipeline explanation available as well as the explanation of the source code tree (that includes nested explanations of important directories and files) available ​here.

Addition of Types for use by Haskell

./compiler/types/TyCon.lhs

TyCons represent type constructors. There are @data declarations, @type synonyms, @newtypes and class declarations (@class). We will need to modify this to add a proper type constructor.

Prelude uses this type constructor in ./compiler/prelude/TysPrim.lhs

Modifications to the compiler to add primtype / primop to Prelude

./compiler/prelude/PrelNames.lhs

./compiler/prelude/TysPrim.lhs

./compiler/prelude/primops.txt.pp

Addition of a type here (primtype) to operate on

Defines the primops that are associated with the types that are defined

For primops defined here that are inline, modify compiler/codeGen/CgPrimOp.hs

Modifications to add support to the compiler for the new types. Look to individual files for the portion of the compiler that is being modified. The binaries generated out of this branch are called from the ghc/ binaries.

./compiler/codeGen/CgCallConv.hs

./compiler/codeGen/CgPrimOp.hs

./compiler/codeGen/CgUtils.hs

./compiler/codeGen/SMRep.lhs

./compiler/codeGen/StgCmmLayout.hs

./compiler/codeGen/StgCmmPrim.hs

More modifications to the Cmm portion of the compiler chain.

./compiler/cmm contains the code that inputs STG and outputs Cmm

./compiler/cmm/CmmExpr.hs

./compiler/cmm/CmmType.hs

./compiler/cmm/CmmUtils.hs

./compiler/cmm/OldCmm.hs

./compiler/cmm/StackColor.hs

Modifications to the LLVM code generator

Generating the human readable LLVM code (.ll) occurs in the compiler/llvmGen code. It receives Cmm and turns it around to LLVM bytecodes through human readable LLVM code. A "simple" use of LLVM vector instructions using floats is shown at the SIMD Vector Example In LLVM page

compiler/llvmGen/Llvm/AbsSyn.hs - no changes, describes the abstract structure of an LLVM program

compiler/llvmGen/Llvm/PpLlvm.hs - no changes, this is the module that does a Pretty Print of LLVM I(ntermediate) R(epresentation), it still uses generic constructs described in Llvm.AbsSyn (where no changes are necessary), Llvm.Types (where changes are needed)

compiler/llvmGen/Llvm/Types.hs - Describes the LLVM Basic Types and Variables. Changes are necessary to add Vectors. LMVector Int LlvmType will be added (eventually constraints on the width may need to be added. "fadd" is already present, as is "add" and other operations. Most likely, additional operations will have to be added as the code generated is slightly different for, say, a float vector since a series of instructions have to be executed for each "add", even though the basic "fadd" structure remains the same.

compiler/llvmGen/LlvmCodeGen/Base.hs, no changes appear necessary here. These seem to be primarily various Label and Environment handling items ... primarily structural.

compiler/llvmGen/LlvmCodeGen/CodeGen.hs, changes will most likely be necessary here for various operators. This is the primary location where Cmm is converted to LLVM operators ... for example, MO_F_Add with a parameter is converted to LM_MO_FAdd here: MO_F_Add _ -> genBinMach LM_MO_FAdd). In the event that additional operators are added (MO_VF_Add for example), this will definitely have to be modified.

Modify autoconf

Determining if a particular hardware architecture has SIMD instructions, the version of the instructions available (SSE, SSE2, SSE3, SSE4 or an iteration of one of those), and consequently the size of vectors that are supported occurs during the configuration step on the architecture that the build will occur on. This is the same time that the sizes of Ints are calculated, alignment constants, and other pieces that are critical to GHC.

Backing up from the results to the location that the changes are introduced, the current alignment and primitive sizes are available in ./includes/ghcautoconfig.h, here is a sample:

These are constructed from mk/config.h* that are generated by configure.ac and autoheader. The configure.ac (or a related file) should be able to be sufficiently modified to determine if SSE is available and, consequently, the vector size that can be operated on and (later) how many pieces of data can be operated on in parallel (determined by the operation). SSE had an MMX register size of 64-bits and all later SSE versions (2 and above) have a register size of 128 bits. This implies that any type that is 32-bits can have 4 pieces of data calculated against in a single instruction.

There is an example of configure.ac modifications to detect SSE availability available on the web, the primary body of the check is as follows (xmmintrin.h contains the SSE instruction set):

Once the mk/config.h is modified with the above, the includes/ghcautoconf.h is modified during the first stage of the GHC build process. Once includes/ghcautoconf.h is modified, the _USE_SSE constant is available in the Cmm definitions (next section).

It should be noted, that since the overall goal is to let the LLVM handle the actual assembly code that does vectorization, it's only possible to support vectorization up to the version that the LLVM supports.

Add new MachOps to Cmm code

It may make more sense to add the MachOps to Cmm prior to implementing the PrimOps (or at least before adding the code to the CgPrimOp.hs file). There is a useful ​Cmm Wiki Page available to aid in the definition of the new Cmm operations.

Modify compiler/cmm/CmmType.hs to add new required vector types and such, here is a basic outline of what needs to be done:

Some existing Cmm instructions may be able to be reused, but there will have to be additional instructions added to account for vectorization primitives. This will help keep the SIMD / non-SIMD code generation separate for the time being until we have it working.

Add new PrimOps

Adding the new PrimOps is relatively straight-forward, but a substantial number of LOC will be added to achieve it. Most of this code is cut/paste "like" with minor type modifications.

Add the following primops associated with the above primtypes. The ops come in groups associated with the types above, for example for IntVec#’s we get the following family for the “plus” operation alone:

plusInt8Vec# :: Int8Vec# -> Int8Vec# -> Int8Vec#

plusInt16Vec# :: Int16Vec# -> Int16Vec# -> Int16Vec#

plusInt32Vec# :: Int32Vec# -> Int32Vec# -> Int32Vec#

plusInt64Vec# :: Int64Vec# -> Int64Vec# -> Int64Vec#

Repeat this for the following set of operations on IntVec#’s of various lengths, note that the signatures are summarized informally in parentheses behind the operation:

Do NOT Modify ./compiler/prelude/PrimOp.lhs (actually, ./compiler/primop-data-decl.hs-incl) to add the new PrimOps (VIntQuotOp, etc...), this will be generated based on the primops.txt.pp modifications

Modify ./compiler/codeGen/CgPrimOp.hs, code for each primop (above) must be added to complete the primop addition.

The code, basically, links the primops to the Cmm MachOps (that, in turn, are read by the code generators)

It looks like some Cmm extensions will have to be added to ensure alignment and pass vectorization information onto the back ends, the necessary MachOps will be determined after the first vertical stack is completed (using the "Double" as a model). There may be some reuse from the existing MachOps. There is some discussion to these extensions (or similar ones) on the original ​Patch 3557 Documentation

Example of modification to ./compiler/prelude/primops.txt.pp to add one of the additional Float operations:

The above, after compilation, adds the following to the ./compiler/prelude/PrimOp.lhs file:

| FloatVectorAddOp
| FloatVectorMultOp

Modify LLVM Code Generator

Take the MachOps in the Cmm definition and translate correctly to the corresponding LLVM instructions. LLVM code generation is in the /compiler/llvmGen directory. The following will have to be modified (at a minimum):

/compiler/llvmGen/Llvm/Types.hs - add the MachOps from Cmm and how they bridge to the LLVM vector operations

/compiler/llvmGen/LlvmCodeGen/CodeGen.hs - This is the heart of the translation from MachOps to LLVM code. Possibly significant changes will have to be added.

Remaining /compiler/llvmGen/* - Supporting changes

At this point, CodeGen is not modified, though it will likely have to be eventually. Types.hs has a new LMVector type added to support vectors. As the operations on vectors are the same as all LLVM types (for float vectors use fadd, etc...), I have not made changes to the operators yet (though I'm guessing I will have to eventually). Here is the diff of changes to Types.hs:

Modify Native Code Generator

Unfortunately, the native code generator will also have to be recompiled. The GHC compilation depends on a 6.x version of GHC, before native LLVM code generation was built into the HEAD (so simply modifying the mk/build.mk file to go to -fllvm does not work).

For x86 Native Code Generation, locate the ./compiler/nativeGen/X86/CodeGen.hs file and modify it appropriately. For the example above, simply adding a conversion from MO_VF_Add to the equivalent non-vector add is sufficient.

Example: Demonstrate SIMD Operation

Once the Code Generator, PrimOps and Cmm are modified, we should be able to demonstrate performance scenarios. The simplest example to use for demonstrating performance is to time vector additions and multiplications using the new vectorized instruction set against a similar addition or multiplication using another PrimOp.

The following two simple programs should demonstrate the difference in performance. The program using the PrimOps should improve performance approaching 2x (Doubles are 64bit and SSE provides two 64bit registers).

Simple usage of the new instructions to add to vectors of doubles:

Question: How does one create one of the new PrimOp types to test prior to testing the vector add operations? This is going to have to be looked at a little ... the code should basically create a vector and then insertDoubleVec# repeatedly to populate the vector. Without the subsequent steps done, this will have to be "hand" done without additional operations defined. Here is the response from Manuel to expand on this: I am not quite sure what the best approach is. The intention in LLVM is clearly to populate vectors using the 'insertIntVec#' etc functions. However, in LLVM you can just use an uninitialised register and insert elements into a vector successively. We could provide a vector "0" value in Haskell and insert into that. Any other ideas?

Once we've shown there is speed-up for the lower portions of the compiler and have quantified it, the upper half of the stack should be optimized to take advantage of the vectorization code that was added to the PrimOps and Cmm. There are two primary locations this is handled, in the compiler (compile/vectorize) code that vectorizes modules post-desugar process. This location handles the VECTORISE pragmas as well as implicit vectorization of code. The other location that requires modification is the Vector library itself.

Modify the Vector library /libraries/vector/Data to make use of PrimOps where possible and adjust VECTORISE pragmas if necessary

Modify the existing Vector code

We will likely also need vector versions of array read/write/indexing to process Haskell arrays with vector operations (this may need to go into compiler/vectorise)

Use the /libraries/vector/benchmarks to test updated code, look for

slowdowns - vector operations that cannot benefit from SIMD should not show slowdown

speedup - all performance tests that make use of maps for the common operators (+, -, *, etc..) should benefit from the SIMD speedup

Modify the compiler/vectorise code to adjust pragmas and vectorization post-desugar process. These modifications may not need to be made on the first pass through the code, more evaluation is necessary.

/compiler/vectorise/Vectorise.hs

/compiler/vectorise/Vectorise/Env.hs

/compiler/vectorise/Vectorise/Type/Env.hs

Once the benchmarks show measurable, reproducible behavior, move onto the DPH libraries. Note that a closer inspection of the benchmarks in the /libraries/vector/benchmarks directory is necessary to ensure they reflect code that will be optimized with the use of SIMD instructions. If they are not appropriate, add code that demonstrates SIMD speed-up appropriately.

Modify DPH Libraries

The DPH libraries have heavy dependencies on the previous vectorization modification step (modifying the Vector libraries and the compiler vector options and post-desugar vectorization steps). The DPH steps should not be undertaken without significant performance improvements illustrated in the previous steps.

The primary changes for DPH are in /libraries/dph/dph-common/Data/Array/Parallel/Lifted/*

VECTOR SCALAR is also heavily used in /libraries/dph/dph-common/Data/Array/Parallel/Prelude, these should be inspected for update as well (Double.hs, Float.hs, Int.hs, Word8.hs)

Modify pragmas as necessary based on changes made above

Note to Self: Determine if the VECTORISE pragmas need adjustment or enhancement (based on previous steps)

Ensure Remaining Code Generators Function Properly

There are really two options on the remaining code generators:

Modify each code generator to understand the new Cmm instructions and restore them to non-vectorized instructions

Add a compiler step that that does a pre-pass and replaces all "length = 1" vectors and operations on them by the corresponding scalar type and operations

The latter makes sense in that it is effective on all code generators, including the LLVM code generator. Vectors of length = 1 should not be put through SIMD instructions to begin with (as they will incur substantial overhead for no return).

To make this work, a ghc compiler flag must be added that forces all vector lengths to 1 (this will be required in conjunction with any non-LLVM code generator). A user can also use this option to turn off SIMD optimization for LLVM.

Add the ghc compiler option: --vector-length=1

Modify compiler/vectorise to recognize the new option or add this compiler pass as appropriate

Reference Documentation

Reference Discussion Threads

From: Manual Chakravarty
Q: Should the existing pure Vector libraries (/libraries/vector/Data/*) be modified to use the vectorized code as a first priority, wait until DPH (/libraries/dph/) is modified, or leave the Vector library as is?
A: The DPH libraries ('dph-*') are based on the 'vector' library — i.e., for DPH to use SIMD instruction, we must modify 'vector' first.
Q: How does one create one of the new Vector Types in a Haskell program (direct PrimOp?, for testing ... let x = ????)
A: I am not quite sure what the best approach is. The intention in LLVM is clearly to populate vectors using the 'insertIntVec#' etc functions. However, in LLVM you can just use an uninitialised register and insert elements into a vector successively. We could provide a vector "0" value in Haskell and insert into that. Any other ideas?
A: I just realised that we need vector version of the array read/write/indexing operations as well to process Haskell arrays with vector operations.
Q: One discussion point was that the "Vector Lengths" should be "Set to 1" for non LLVM code generation, where does this happen? On my first survey of the code, it seems that the code generators are partitioned from the main body of code, implying that each of the code generators will have to be modified to account for the new Cmm MachOps? and properly translate them to non-vectorized instructions.
A: Instead of doing the translation for every native code generator separately, we could have a pre-pass that replaces all length = 1 vectors and operations on them by the corresponding scalar type and operation. Then, the actual native code generators wouldn't need to be changed.
A: The setting of the vector length to 1 needs to happen in dependence on the command line options passed to GHC — i.e., if a non-LLVM backend is selected.
Q: Can we re-use any of the existing MachOps? when adding to Cmm?
A: I am not sure.